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Has Gulf of Mexico cleanup turned a corner?

Visible oil is more and more sparse on the surface of the Gulf of Mexico

Weather, dispersants and oil skimmers have cleared the surface

Waters have been reopened to fishing, and seafood safety tests are being implemented

How much oil remains under the sea is unclear

(CNN) -- With BP's ruptured oil well in the Gulf of Mexico apparently securely capped, the sparseness of visible slicks is prompting the question, "Has the Gulf cleanup turned the corner?"

On the surface, the answer is "yes," but if you dig deeper, the truth is murky.

Atop the Gulf

After touring the spill scene Sunday, Coast Guard Rear Adm. Paul Zukunft, the federal on-scene coordinator, said that oil left on the surface is breaking down "very quickly" naturally, now that the gushing crude has been stifled since July 15.

Zukunft said he saw only one large patch of emulsified oil, about 12 miles off Grand Isle, Louisiana, during his six-hour aerial tour. No oil could be seen in Louisiana's Lake Borgne, Lake Pontchartrain or Chandeleur Sound, and only a light sheen was visible in other parts of the Gulf.

"The oil is basically approaching the end of its life cycle," he said.

Steve Murawski, the National Oceanic and Atmospheric Administration's chief scientist for fisheries, delved deeper into some of Mother Nature's effects Monday. He said the "really hot" water on the surface and the scorching air temperatures were helping eliminate the surface oil. "Things break down in weather really fast," he said.

"It's obvious what's going on at the surface. The big issue is what's trapped in the marsh," Murawski said. The oil in the marshes and beaches was ferried by winds, which tend to blow from the south and southeast toward the shore, he said.

Retired Coast Guard Adm. Thad Allen, who's leading the federal government's response team, told CNN's "The Situation Room" on Friday, "I am not ready to declare victory, nor should anybody, because we have cleanup in the marsh areas that are affected. But we are certainly starting to gain a little upper hand here."

Fishing for answers

On Thursday, Vice President Joe Biden visited coastal Alabama to announce that the National Oceanic and Atmospheric Administration and the Food and Drug Administration were reopening more than 25,000 square miles of the Gulf to fishing, with the ability to certify that all the seafood derived from those waters would be safe to eat.

"What we are talking about here is almost a third of the entire closed area in federal waters will be opened, and we're going to continue to work to see the rest of it is opened as soon as it can be safely done, as soon as we can guarantee that the fish coming out of those waters are edible and safe as they were before the spill," Biden said.

Murawski said the reopened fishing terrain is "fairly far south of the well." It was originally declared off-limits because of oil sheens on the surface water, but recent water samples passed safety standards, he said.

He said seafood testing will have two major components. The first, a "sensory test," requires testers to fillet fish to see whether they can smell oil when the fish is opened. "At 1 part per million, you can smell it," he said, adding that 1 part per million is "not problematic" health-wise.

"Oil does not necessarily bio-magnify into larger things," Murawski said. Fish, dolphins and other larger sea animals process and excrete oil. However, he said, consumers "need to be more careful with oysters and shrimp."

The second test is a chemical test, in which researchers check to see whether the sea life exceeds standards for 12 compounds known as polycyclic aromatic hydrocarbons.

The Centers for Disease Control says that in animal studies, polycyclic aromatic hydrocarbons have caused reproduction problems, birth defects, low body weight and harmful effects on the skin, body fluids and ability to fight disease after both short- and long-term exposure. The effects of the chemicals on humans are unclear, the CDC says, but the Department of Health and Human Services has determined that some polycyclic aromatic hydrocarbons "may reasonably be expected to be carcinogens."

"We should be concerned about anything in the marine environment that's not a natural product," Murawski said.

He added that the Food and Drug Administration says that given the volume and application rate of dispersants, as well as their dilution, they should not be toxic to humans, "but there are concerns about marine life." In certain concentrations, Murawski said, dispersants can kill free-floating eggs and larvae. It's unclear how the chemicals affect the animal population under sea.

Under the sea

"There is an as yet imprecise estimate ... how much [oil] is trapped subsurface," Murawski said. NOAA spokeswoman Rachel Wilhelm said scientists are still refining those calculations.

Murawski described the water 3,300 to 4,300 feet under the Gulf's surface: "If you saw it in a glass, it would look like seawater with very small droplets," he said.

The Gulf's water contains 1 to 2 parts of oil per million close to the wellhead and dissipates farther away, he said. By comparison, samples taken about 140 miles to the southeast of the well contained about 0.3 to 0.4 parts per million, Murawski said, which is within the naturally occurring range of zero to 0.5 parts per million in an area known for natural seepage through the floor.

He said that evidence makes it appear the dispersants used to help clean up the spill are doing their job.

"The whole point of dispersants is to break it down in small bits so it can be consumed by bacteria, but not at such a rate that it starves the ecosystem of oxygen," Murawski said. He pointed out that researchers looked farther down the wellhead into deeper waters, where temperatures sank to about 41 degrees Fahrenheit. At those cold temperatures, he said, bacterial concentrations were rising.

Murawski said the rate of dispersant use has been keyed toward regulation of oxygen levels in the water. He said crews need to be very careful about the oxygen levels, if bacteria eat the oil too quickly.

But he admitted no one has ever really accounted for what happens to dispersants in deep water.